A tennis racket comprises a handle attached to a frame, which frame is provided with stringing in the form of a grid having a pair of mutually perpendicular arrays of strands. In the prior art, such strands are usually of generally circular cross-section and the strands of the pair of arrays are usually woven so that each strand of each array passes on opposite sides of each adjacent pair of strands in the other array.
Thus, the faces of the grid formed by arrays of strands present a foraminous surface to a resilient tennis ball which is impacted thereby. Such foraminous surface tends to grip the surface of the tennis ball and enables various desired spins to be imparted to the tennis ball depending on the relative movement which occurs between such surface and the ball during impact.
However, the force of the impact is distributed over the convex surfaces of the portions of strands in the arrays forming the grid which are impacted by the ball, thereby tending to reduce the frictional forces which impart the desired spin to the ball. Furthermore, the contact between the strands of one array and the strands of the other array is essentially a single point contact between the convex generally circular exterior surfaces of the strands. Although such contact will provide high frictional forces, it will not prevent the rolling of the convex generally circular surfaces of strands on each other. Such rolling will not only tend to attenuate the forces which would otherwise impart spin to an impacted tennis ball but will also tend to displace the strands from their desired location in the arrays.
It has been proposed in the prior art that the above problems can be reduced by forming the strands of the arrays of the grid of a tennis racket with generally flat side surfaces to produce an angular cross-section defining at least two edges and with the side surfaces and edges being helically shaped. (See U.S. Pat. No. 4,005,863, issued to Dana R. Henry on Feb. 1, 1977.) However, although the approach may provide some slight increase in the frictional forces between the strands and the ball upon impact over that provided by strands of circular cross-section, it tends to reduce the frictional forces between the strands themselves by tending to provide surface-to-surface contact instead of point-to-point contact. Thus, the strands of the arrays will slide on each other, resulting in serious dislocation of the strands of the arrays in use as well as attenuation of forces tending to impart spin to the ball. Furthermore, any increase in frictional forces between the strands and the ball upon impact due to the ridges provided by the angular cross-section of the strands in accordance with this prior art approach will tend to be negated by their helical form and flat side surfaces. The helical form will result in the ball impacting on generally flat surfaces equal in extent to the desired ridged surfaces and a resulting equalization in frictional forces between the strands and an impacted ball.
It is the object of this invention to provide an improvement overcoming the deficiencies of the prior art as set forth above.